Condensed matter spectroscopy


Superresolved fluorescence microscopy on microgels and microgel systems

  Superres Microgels Copyright: © Dominik Wöll

The elucidation of the structure of compartmentalized microgels and microgel structures is challenging since they do not possess strong contrast for electron microscopy, and since classical fluorescence microscopy fails due to the fact that their structures are significantly smaller than the diffraction limit of optical light. However, we are now using modern superresolved fluorescence microscopy methods such as dSTORM (direct stochastic optical reconstruction microscopy) to fill this gap of structural in situ imaging. Thus, different types of microgel architectures can be distinguished and analyzed either by direct 3D measurement or by analysis of 2D projections. For this purpose, we developed the software tool SoMaCoFit.

This project is part of the High Resolution Fluorescence Microscopy Competence Center FLAMENCO within the RWTH profile area „Molecular Science and Engineering“. An introductory movie can be found here.


Diarylethenes as photoswitches for superresolved fluorescence microscopy in soft matter

  Photoswitches Copyright: © Dominik Wöll

Superresolved fluorescence microscopy methods have been frequently applied to biological samples. Adapting these methods to materials science, and in particular to apolar polymer systems, is a challenging task due to the need for appropriate dyes and labeling strategies. Most superresolution imaging techniques are based on the switching of fluorophores between a fluorescent and a non-fluorescent state. Diarylethenes, which can be interconverted between an open- and a closed-ring form can be used as key elements of various light-driven molecular switches. We investigate the application of different diarylethenes derivatives with high fluorescence quantum yields in their fluorescent closed form and with suitable photo kinetics for photoactivated localization microscopy (PALM) and superresolution optical fluctuation imaging (SOFI) in polymer systems. This way, we could nanoscopically visualize of self-assembled block copolymer structures (see image).


Single molecule dynamics in thin polymer layers close to the glass transition

  Single molecule dynamics Copyright: © Dominik Wöll

We developed a new heating table for microscopes with which we measured the translational diffusion coefficients of single perylene diimide molecules in thin polystyrene films up to temperatures of 150 °C. We analyzed the distributions of diffusion coefficients and found that the number of mobile molecules depends strongly on film thickness, see figure below. This behaviour can be modeled with Monte Carlo random walk simulations assuming a reduced glass transition temperature and an increased residence probability of dye molecules at the polymer surface.


Fluorescence techniques to probe translational and rotational motion

  Fluorescence techniques Copyright: © Dominik Wöll

We analyze the motion of single molecules and single nanoparticles in order to get new insights into dynamic processes on the molecular scale and to elucidate static and dynamic heterogeneities. The results of our studies are not only of fundamental importance but also essential for the development of new materials. Within the last years, we developed a concept to detect translational and rotational diffusion over a broad range of ca. 12 orders of magnitude combining different fluorescence microscopy methods. This allows for mobility measurements from free diffusion in low-viscous solvents down to very slow single molecule motion in highly-viscous polymers. The methods and their suitability for different diffusion coefficients are shown in the Figure below.


Photo-switchable multichromophoric systems for single molecule fluorescence microscopy

  Photo-switchable multichromophoric systems Copyright: © Dominik Wöll

We synthesized, characterized, and applied a novel photocleavable energy-transfer dyad to single-molecule fluorescence microscopy. After photocleavage, a combination of independent two-color single-molecule tracking and analysis of single-molecule energy-transfer efficiencies allows the determination of the temporal evolution of the relative distances between both fragments from the nano- to the micrometer scale. This gives access to a broad range of diffusion coefficients.


Scaling of translational and rotational diffusion in polymerizing solutions


Diffusional changes during the bulk radiacal polymerization of styrene and MMA are investigated using Single Molecule Fluorescence Spectroscopy and Microscopy Methods. Fluorescence correlation spectroscopy with full correlation allowed us to follow and compare changes in translational and rotational diffusion:

  Scaling Copyright: © Dominik Wöll

It could be shown that the relative decrease in rotational motion was independent of the size of the fluorescent probe whereas significant differences were observed for translational diffusion. Here, the motion of large dyes becomes more restricted than expected when assuming Stokes-Einstein-Debye behavior. The observations were connected to a decreasing size of meshes formed by polymer chains with increasing polymer conversions.


Mobility of nanoparticles

  Mobility Copyright: © Dominik Wöll

We aim to use fluorescence microscopy to follow the motion of single polymer and clay nanoparticles in different systems of practical importance. In collaboration with the group of Prof. Stefan Mecking (department of chemistry, University of Konstanz) we synthesized semi-crystalline polyethylene nanoparticles that were predominately labeled with one chromophore. The chromophore exhibits a preferential orientation with respect to the axes of the anisotropic nanoparticles as determined using defocussed fluorescence widefield microscopy. This enables parallel investigation their transitional and rotational motion.


Novel single molecule tracking approaches

  single molecule tracking approaches Copyright: © Dominik Wöll

Determination of molecule positions with high accuracy and correct connection of the determined positions to single molecule tracks remains a challenging task with, so far, no universal solution. In cooperation with Dr. A. Karrenbauer from MPI Saarbrücken we developed a novel tracking model using a top-down polyhedral approach which can be implemented effectively using standard linear programming solvers. Our algorithm reliably connects positions to trajectories and works very well even for high spot densities and frequent fluorescence blinking, the two main challenges of single molecule tracking.